Parsing iCal was far more performant than parsing JSON as your parser could simply start at the first byte, and follow the spec — whereas a JSON parser would first need to wait for ten years before JSON was invented.
The original comment implied that parsing JSON was less efficient (either in terms of coding required, or the resulting CPU use) than parsing iCal.
While it is true that dealing with json by hand is going to be more work than just importing an iCal library, I'm comparing apples to apples and suggesting dealing with json "by hand" is no more faff than dealing with iCal "by hand", and that dealing with either via a good library equally gives no benefit to iCal.
Most data formats used in high performant scenarios, for example in finance, are very basic for a reason. You also get the benefit of being able to start processing the file immediately line by line, instead of having to read and parse the entire file.
